Method and apparatus for channel estimation

ABSTRACT

The invention relates to DVB-T system, and in particular, to a channel estimation method for OFDM symbols. A plurality of symbols are received to generate a pilot response. A finite impulse response is generated from the pilot response. A coefficient table is selected based on the characteristics of the finite impulse response. The channel is estimated by interpolating the pilot response based on the coefficient table.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of pending U.S. patent applicationSer. No. 11/279,916, filed Apr. 17, 2006, the entirety of which isincorporated by reference herein.

BACKGROUND

The invention relates to OFDM communication systems, and in particular,to a channel estimation method for DVB-T system.

FIG. 1 a shows conventional DVB-T symbols. The horizontal rows are DVB-Tsymbols comprising a plurality of sub-carriers, arranged in verticaltime direction. The white nodes frequency direction interpolator 110carry data, and the black nodes such as 210 a, 210 b and 210 c, arepilots. The distribution of pilots varies in time direction,periodically repeating every four symbols. Conventionally, pilots areutilized to estimate the channel. For example, a pilot response at thefifth row (t=5) is generated by interpolation of adjacent pilots in timedirection. The pilots 210 a and 230 a interpolate the pilot 220 a, thepilots 210 b and 230 b interpolate the pilot 220 b, and the pilots 210 cand 230 c interpolate the pilot 220 c. The interpolation may be a linearinterpolation, and is also referred to as a time direction interpolation(TDI).

FIG. 1 b shows a pilot response obtained from the DVB-T symbols ofFIG. 1. The pilots H₀ to H₅ are shown at an interval. The channels forthe sub-carriers therebetween, such as H_(a) and H_(b), are estimated byfrequency direction interpolation. Conventionally, the frequencydirection interpolation may be accomplished through various algorithms,such as linear interpolation, second order interpolation, third orderinterpolation, bi-linear interpolation and fixed finite impulse response(FIR) interpolation. The estimated channel is utilized to restoretransmitted data in the sub-carriers in an equalization process,therefore an efficient channel estimation method can improve the DVB-Treceiver performance.

SUMMARY

An exemplary channel estimation method for an OFDM receiver is provided.A plurality of symbols are received to generate a pilot response. Afinite impulse response is generated from the pilot response. Acoefficient table is selected based on the characteristics of the finiteimpulse response. The channel is estimated by interpolating the pilotresponse based on the coefficient table.

The generation of the pilot response may accomplished by collecting allpilots distributed in the plurality of symbols, and interpolating theadjacent pilots in time direction to determine an element in the pilotresponse. Generation of the finite impulse response may accomplished byperforming IFFT on the pilot response.

The finite impulse response is further filtered to eliminate componentsunder a predetermined threshold to generate a window, and a width and aposition of which are determined.

A plurality of coefficient tables are further provided, each adaptablefor a specific window width. One coefficient table is selected based onthe window width to estimate the channel.

A plurality of coefficient vectors are further generated from thecoefficient table based on the window position. The coefficient tablescomprise a plurality set of real numbers, and the coefficient vectorsare generated by rotating the real numbers by an angle corresponding tothe window position. The coefficient vectors are multiplied withelements in the pilot response to rebuild the channel.

Another embodiment of the invention provides a channel estimatorperforming the described method.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description, given by way of example and notintended to limit the invention solely to the embodiments describedherein, will best be understood in conjunction with the accompanyingdrawings, in which:

FIG. 1 a shows conventional DVB-T symbols;

FIG. 1 b shows a pilot response obtained from the DVB-T symbols of FIG.1;

FIG. 2 is a block diagram of a channel estimator according to anembodiment of the invention;

FIG. 3 shows a finite impulse response generated from the pilotresponse;

FIG. 4 shows an embodiment of the frequency direction interpolation; and

FIG. 5 is a flowchart of the channel estimation method according to theinvention.

DETAILED DESCRIPTION

A detailed description of the present invention is provided in thefollowing.

FIG. 2 is a block diagram of a channel estimator according to anembodiment of the invention. A novel interpolation method is provided. Atime direction interpolator 102 receives a plurality of symbols from afront end such as a FFT unit (not shown), with frame synchronized andguard interval removed, such that a symbol array shown as FIG. 1 a isobtained, comprising a plurality of consecutive symbols. The pilots inthe symbols are scattered as the specification defines, distributedperiodically to facilitate channel estimation. The time directioninterpolator 102 performs a time direction interpolation to generate apilot response comprising the interpolated pilots as shown in the fifthsymbol in FIG. 1 a. A leaky integrator may be utilized to perform thetime direction interpolation, with pilot response an averaged result.For a DVB-T 2K mode, a symbol may comprise 1705 effective sub-carriers,⅓ thereof interpolated to form the pilot response since the pilots arescattered every 3 sub-carriers. The elements in the pilot response varywith mode. For example, including the first pilot, the elements are 569in the 2K mode, 1137 in the 4K mode, and 2273 in the 8K mode. The pilotresponse represents a preliminary channel, and a frequency directioninterpolation is required to rebuild a complete channel for restorationof every sub-carrier in the symbols. An IFFT unit 104 is coupled to thetime direction interpolator 102, performing IFFT to generate a finiteimpulse response from the pilot response. In the IFFT unit 104, thefinite impulse response is filtered to eliminate components under apredetermined threshold, such that a window is generated. The thresholdmay be a fixed value, or a ratio of the maximum magnitude in the finiteimpulse response. The remaining impulses after filtering form a window,and the window width t₁ and window position t₀ are determined. Thewindow width t₁ is the duration from the first impulse to the lastimpulse, and the window position t₀ is the time index of the center ofthe window width t₁. The t₀ and t₁ are sent to the DSP 108. The DSP 108is coupled to the IFFT unit 104 and a memory 106. The memory 106provides a plurality of coefficient tables each adaptable for a specificwindow width, and the DSP 108 selects one of the coefficient tablesbased on the window width t₁ of the finite impulse response. Thecoefficient tables are programmable sets of real numbers specificallydesigned for the frequency direction interpolation. After determining acoefficient table according to the window width, the real numberstherein are rotated by the DSP 108 with an angle corresponding to thewindow position t₀, thus a plurality of coefficient vectors aregenerated. In this way, the coefficient vectors are a function of thewindow width t₁ and window position t₀, and the frequency directioninterpolation can be performed therewith. The frequency directioninterpolator 110 is coupled to the time direction interpolator 102 andthe DSP 108, multiplying the coefficient vectors with elements in thepilot response to rebuild the channel.

FIG. 3 shows a finite impulse response generated from the pilotresponse. As described, the pilot response may comprise differentnumbers of elements in different modes, and an IFFT is performed togenerate the finite impulse response. Specifically, to facilitate theIFFT implementation, the number of elements selected to perform the IFFTmay be an exponent of 2, such as 2^(M) where M is an integer. The IFFTmay be performed multiple times over a period to obtain averaged resultsamong a plurality of symbols, thus the finite impulse response isaveraged. An autoregression moving average algorithm may also be appliedto keep the finite impulse response updated. A threshold V_(th) is setto filter noise components. As described, the threshold V_(th) may be afixed value or a ratio of the maximum magnitude in the finite impulseresponse. The remaining impulses exceeding the threshold V_(th) aredeemed to be valid channel paths, and a window 302 is formed therefrom.The duration from the first impulse to the last impulse in the window302, is determined to be the window width t₁. The time index of themiddle of the window is determined to be the window position t₀. Thewindow width is also referred to as a channel length, and the windowposition is the channel position. One of the coefficient tables isselected based on the window width, such that the interpolation error inthe frequency direction interpolator 110 can be minimized.

FIG. 4 shows an embodiment of the frequency direction interpolation. Aplurality of coefficient vectors are provided to perform theinterpolation. For example, three coefficient vectors are provided asW_(a), W_(b) and W_(c):

-   -   W_(a)={W_(a1), W_(a2), W_(a3), W_(a4), . . . , W_(aN)}    -   W_(b)={W_(b1), W_(b2), W_(b3), W_(b4), . . . , W_(bN)}    -   W_(c)={W_(c1), W_(c2), W_(c3), W_(c4), . . . , W_(cN)}    -   where the coefficients therein can be described as:    -   W_(ak)=e^(j(3M−3k−1)Θ)R_(ak) k=1 to N    -   W_(bk)=e^(j(3M−3k)Θ)R_(bk) k=1 to N    -   W_(ck)=e^(j(3M−3k+1)Θ)R_(ck) k=1 to N

The number N can be 2M+1, where M is an integer not exceeding ½ of theelement numbers in the pilot response, and the angle Θ is obtained bythe window position t₀:

$\theta = \frac{2\; {\pi \cdot t_{0}}}{L}$

wherein L is a predetermined value corresponding to the FFT size whenreceiving the symbol. The R_(ak), R_(bk) and R_(ck) with k=1 to N, arereal numbers provided in the selected coefficient table. Thus, thecoefficients in the coefficient vectors are obtained by the DSP 108transforming the real numbers in the selected coefficient table based onthe angle Θ.

As shown in FIG. 4, the n-th group of the pilot elements comprisesA_(n), B_(n) and C_(n), where B_(n) is the n-th pilot and the A_(n) andC_(n) are adjacent channels to be determined. Thus, the channelcorresponding to the n-th group of sub-carriers can be estimated as:

A _(n) =W _(a1) B ₁ +W _(a2) B ₂ +W _(a3) B ₃ + . . . +W _(aN) B _(N)

B _(n) =W _(b1) B ₁ +W _(b2) B ₂ +W _(b3) B ₃ + . . . +W _(bN) B _(N)

C _(n) =W _(c1) B ₁ +W _(c2) B ₂ +W _(c3) B ₃ + . . . +W _(cN) B _(N)

Note that the n-th pilot B_(n) itself is updated from the original knownvalue, and the equation can be generalized to the forms of:

$A_{n} = {\sum\limits_{k = {- M}}^{M}{B_{n + k}W_{ak}}}$$B_{n} = {\sum\limits_{k = {- M}}^{M}{B_{n + k}W_{bk}}}$$C_{n} = {\sum\limits_{k = {- M}}^{M}{B_{n + k}W_{ck}}}$

In this way, the channel corresponding to every sub-carrier in a symbolcan be obtained from the interpolation equation. After the channelestimation in the frequency direction interpolator 110, the symbols aresent to an equalizer for further processes.

FIG. 5 is a flowchart of the channel estimation method according to theinvention. In step 502, a plurality of symbols are received to generatea pilot response. In step 504, a finite impulse response is generatedfrom the pilot response. In step 506, a coefficient table is selectedbased on the characteristics of the finite impulse response. In step508, the channel is estimated by interpolating the pilot response basedon the coefficient table.

While the invention has been described by way of example and in terms ofpreferred embodiment, it is to be understood that the invention is notlimited thereto. To the contrary, it is intended to cover variousmodifications and similar arrangements (as would be apparent to thoseskilled in the art). Therefore, the scope of the appended claims shouldbe accorded the broadest interpretation so as to encompass all suchmodifications and similar arrangements.

1. A channel estimator for an orthogonal frequency division multiplexer(OFDM) receiver, comprising: a time direction interpolator, receiving aplurality of symbols to generate a pilot response; an inverseFast-Fourier Transform (IFFT) unit, coupled to the time directioninterpolator, generating a finite impulse response from the pilotresponse; a digital signal processing unit, coupled to the IFFT unit,determining a coefficient set based on the characteristics of the finiteimpulse response; and a frequency direction interpolator, coupled to thetime direction interpolator and the digital signal processing unit,estimating the channel by interpolating the pilot response based on thecoefficient set.
 2. The channel estimator as claimed in claim 1,wherein: the time direction interpolator generates the pilot response bycollecting all pilots distributed in the plurality of symbols; and anelement in the pilot response is determined by interpolating theadjacent pilots in time direction.
 3. The channel estimator as claimedin claim 1, wherein the IFFT unit performs IFFT on the pilot response togenerate the finite impulse response.
 4. The channel estimator asclaimed in claim 3, wherein: the IFFT unit filters the finite impulseresponse by eliminating components under a predetermined threshold togenerate a window; and the IFFT unit determines a window width and awindow position from the window.
 5. The channel estimator as claimed inclaim 4, further comprising a memory, providing a plurality ofcoefficient tables each adaptable for a specific window width, whereinthe digital signal processing unit is coupled to the memory, selecting acoefficient table based on the window width for determining thecoefficient set.
 6. The channel estimator as claimed in claim 5, whereinthe digital signal processing unit generates a plurality of coefficientvectors as the coefficient set from the coefficient table based on thewindow position.
 7. The channel estimator as claimed in claim 6,wherein: the coefficient tables comprise a plurality set of realnumbers; and the digital signal processing unit rotates the real numbersby an angle corresponding to the window position, such that thecoefficient vectors are generated.
 8. The channel estimator as claimedin claim 7, wherein the frequency direction interpolator multiplies thecoefficient vectors with elements in the pilot response to rebuild thechannel.
 9. A channel estimation method for execution by an orthogonalfrequency division multiplexer (OFDM) receiver, comprising: receiving bythe OFDM receiver a plurality of symbols to generate a pilot response;generating a finite impulse response from the pilot response;determining a coefficient set based on the characteristics of the finiteimpulse response; and estimating the channel by interpolating the pilotresponse based on the coefficient set.
 10. The channel estimation methodas claimed in claim 9, wherein the generation of the pilot responsecomprises: collecting all pilots distributed in the plurality ofsymbols; and interpolating the adjacent pilots in time direction todetermine an element in the pilot response.
 11. The channel estimationmethod as claimed in claim 10, wherein the generation of the finiteimpulse response comprises performing an inverse Fast-Fourier Transform(IFFT) to the pilot response.
 12. The channel estimation method asclaimed in claim 11, further comprising: filtering the finite impulseresponse by eliminating components under a predetermined threshold togenerate a window; and determining a window width and a window positionfrom the window.
 13. The channel estimation method as claimed in claim12, further comprising providing a plurality of coefficient tables eachadaptable for a specific window width, wherein one coefficient table isselected based on the window width for the determination of thecoefficient set.
 14. The channel estimation method as claimed in claim13, further comprising generating a plurality of coefficient vectors asthe coefficient set from the coefficient table based on the windowposition.
 15. The channel estimation method as claimed in claim 14,wherein: the coefficient tables comprise a plurality set of realnumbers; and the generation of the coefficient vectors comprisesrotating the real numbers by an angle corresponding to the windowposition, such that the coefficient vectors are generated.
 16. Thechannel estimation method as claimed in claim 15, wherein the pilotresponse interpolation comprises multiplying the coefficient vectors byelements in the pilot response to rebuild the channel